The prior art catalysts of this class are classified into stable and ultra-stable types for purposes of distinguishing these catalysts. These prior art catalysts have been formed from faujasite type zeolites, for example, the Y type zeolite (see Breck U.S. Pat. No. 3,130,007), and from X type zeolites (U.S. Pat. No. 2,882,244) by exchange, for example, with H, NH.sub.4, rare earths, and alkaline earth cations or mixtures thereof by ordinary exchange at ambient temperatures. The degree of exchange may be sufficient to reduce the Na content of the zeolite, expressed as Na.sub.2 O to about 3 to 4%. The compounded catalyst has a Na.sub.2 O content of about 1%. The S+ activity, (see further below) of catalysts formed by incorporating the same in a matrix, for example, such as hydrated alumina and clay is in the order of 20%. Matrixes employed in the prior art have included silica gels. Such prior art zeolites are here referred to as Class B zeolites and the catalyst produced as Class B catalysts.
The S+ activity is not materially improved by reducing the Na.sub.2 O content of the zeolite by multiple exchanges down to about 1.75 to about 2%. The low S+ activity is indicative of a zeolite which is to a considerable degree hydrothermally unstable. The exposure of the catalyst, formulated from the zeolite, to steam at 1550.degree. F for 2 hours, referred to as S+ activity has substantially deactivated the catalyst to give the low S+ activity.
While the activity after steaming at the higher temperature of 1550.degree. F is an important characteristic of catalyst of high stability, the activity after a lower temperature steaming at 1450.degree. referred to as M activity is also an important parameter.
The Class B catalysts have, characteristically, M activities in the range of about 60 to about 70 depending on its chemical constitution and concentration of the zeolite in the matrix. The M and S activity were measured by the improved microactivity test described in Oil and Gas Journal issues of 1966, Volume 64, No. 39, pp. 7, 84, and 85, and of Nov. 22, 1971, pp. 60-68.
The prior art has found that by exchanging the zeolite to partially reduce the sodium content of the zeolite and then heating the zeolite by calcining in the presence or absence of steam, the sodium content of the zeolite may be reduced to less than 1% by weight. Such processes are disclosed as U.S. Pat. Nos. 3,293,192, 3,402,996, 3,867,277, 3,281,199, and 3,692,692. Catalysts formed from these zeolites by incorporating the same in a matrix will have an S+ activity in the range of about 45%.
Applicants have discovered an improved method of reducing the sodium content of faujasite type zeolites. The zeolites thus formed have Na.sub.2 O values of from about 2.5% to about 0.5% and catalyst formulated therefrom employing the above matrix have S+ activities in the range of 50 to 60%. (See application Ser. No. 718,167.)
All such zeolites are here termed Class A zeolites and the catalyst formed therefrom as Class A catalysts. They are characterized as of substantially greater hydrothermal stability as represented by its substantially higher S+ activity. M activity of the A type catalysts is in the range of about 40% to about 60% depending on the concentration of the zeolite in the catalysts.
The sodium content of both Class A and Class B catalysts, based on the total catalyst weight on a volatile free basis is below about 1%.